Final answer:
Rats injected with thyroxine do not develop a goiter because thyroxine directly stimulates the production of thyroid hormone, bypassing the need for iodine. Goiter is caused by the overproduction of thyroid-stimulating hormone (TSH) without the formation of thyroid hormone.
Step-by-step explanation:
When rats are injected with thyroxine, a hormone produced by the thyroid gland, it stimulates the production of thyroid hormone in the body.
Thyroid hormone regulates the metabolism and growth of cells and tissues.
In the absence of iodine, the thyroid gland enlarges in a condition called goiter, which is caused by overproduction of thyroid-stimulating hormone (TSH) without the formation of thyroid hormone.
However, when thyroxine is injected, it bypasses the need for iodine and directly stimulates the production of thyroid hormone, preventing the development of goiter in rats.